One example of an axial flow rotary machine is a gas turbine engine of the type used to propel aircraft. An aircraft gas turbine engine typically has a compression section, a combustion section, and a turbine section. An annular flowpath for working medium gases extends axially through the engine. A stator assembly extends axially through the engine and about the working medium flowpath. A rotor assembly extends axially through the sections of the engine and inwardly of the stator assembly. The rotor assembly includes rows of rotor blades which extend outwardly from the rotor across the working medium flowpath in both the turbine and compression sections.
The stator assembly includes an engine case which bounds the working medium flowpath and which is disposed outwardly of the rotor assembly. Rows of stator vanes extend inwardly from the engine case and across the working medium flowpath into proximity with the rotor assembly. The rows of stator vanes are located upstream of the associated row of rotor blades to direct the working medium gases into the downstream row of rotor blades.
One example of a gas turbine engine having a stator assembly and a rotor assembly is shown in U.S. Pat. No. 4,627,233 issued to Baran entitled "Stator Assembly For Bounding The Working Medium Flowpath Of A Gas Turbine Engine". In this construction, the stator assembly includes an outer case and a wall bounding the working medium flowpath which is formed of a plurality of circumferentially extending wall segments, such as the duct 34 and the stator vanes 32. An inner casing in the form of a seal member 86 extends circumferentially between the outer case and the wall elements to form a cooling air chamber outwardly of the inner casing and to block hot gases from the working medium flowpath from the mixing with the cooling air chamber, or air from the cooling air chamber flowing into the working medium flowpath.
The seal member might be formed of relatively thin sheet metal material and formed in a single circumferentially continuous piece. However, the circumferential continuity may result in the seal element being cracked or warped during transient operative conditions of the engine as the outer case changes its diameter at a different rate from the inner case in response to transient thermal conditions of the engine.
A second example of an engine structure employing rotor blades and stator vanes inwardly of an outer case is shown in U.S. Pat. No. 4,431,373 issued to William G. Monsarrat which is entitled "Flow Directing Assembly For A Gas Turbine Engine." In Monsarrat, the compression section of the engine includes an inner case and an outer case or annular sleeve. The outer case is attached to the inner case and positions the segments circumferentially about the rotor assembly during operation of the engine.
The inner case extends axially in the engine outwardly of an annular flowpath for working medium gases. The inner case is formed of a plurality of arcuate segments which are circumferentially adjacent one to the other. Adjacent arcuate segments are spaced circumferentially leaving a circumferential clearance gap G therebetween.
The engine also includes a plurality of cooling tubes which extend circumferentially about the exterior of the outer case. The cooling tubes impinge cooling air under preselected operative conditions of the engine to adjust the radial clearance between the inner case and the rotor assembly. As the cooling air is impinged on the outer case, the outer case contracts and moves inwardly to a smaller diameter, forcing the inner case to a smaller diameter and decreasing the clearance gap G between adjacent arcuate segments and decreasing the radial gap between the rotor assembly and the stator assembly.
The circumferential gap G between adjacent arcuate segments allows the segments to slidably accommodate the inward and outward movmment of the outer case. The gap decreases resistance by the inner case to movement of the outer case; and, it avoids crushing or stretching of the inner case by the outer case at operative conditions where the outer case has a different diameter than the inner case, such as might occur during transient operating conditions of the engine because of differences in thermal expansion between the inner case and the outer case.
As shown in FIG. 4 of U.S. Pat. No. 4,431,373, the adjacent arcuate segments overlap each other to provide sealing. Alternatively, a means for sealing, such as a feather seal, extends circumferentially between the adjacent arcuate segments of the inner case.
The above constructions notwithstanding, scientists and engineers under the direction of Applicant's assignee are working to develop inner cases which have having an acceptable fatigue life while accommodating changes in diameter of the outer case and while blocking the movement of gases.